Sage Scrub Research Articles

Habitat fragmentation in coastal southern California disrupts genetic connectivity in the cactus wren (Campylorhynchus brunneicapillus).

Achieving long-term persistence of species in urbanized landscapes requires characterizing population genetic structure to understand and manage the effects of anthropogenic disturbance on connectivity. Urbanization over the past century in coastal southern California has caused both precipitous loss of coastal sage scrub habitat and declines in populations of the cactus wren (Campylorhynchus brunneicapillus). Using 22 microsatellite loci, we found that remnant cactus wren aggregations in coastal southern California comprised 20 populations based on strict exact tests for population differentiation, and 12 genetic clusters with hierarchical Bayesian clustering analyses. Genetic structure patterns largely mirrored underlying habitat availability, with cluster and population boundaries coinciding with fragmentation caused primarily by urbanization. Using a habitat model we developed, we detected stronger associations between habitat-based distances and genetic distances than Euclidean geographic distance. Within populations, we detected a positive association between available local habitat and allelic richness and a negative association with relatedness. Isolation-by-distance patterns varied over the study area, which we attribute to temporal differences in anthropogenic landscape development. We also found that genetic bottleneck signals were associated with wildfire frequency. These results indicate that habitat fragmentation and alterations have reduced genetic connectivity and diversity of cactus wren populations in coastal southern California. Management efforts focused on improving connectivity among remaining populations may help to ensure population persistence.

Predicting the impact of fire on a vulnerable multi-species community using a dynamic vegetation model

Conservation management under human-induced changes to disturbance requires tools that can balance the needs of multiple species with different life histories and habitat requirements. Despite this urgent conservation need, landscape management typically focuses on single species and rarely includes the influence of disturbance-dependent vegetation transitions on multiple target species. In this paper, we describe a simulation model that achieves these goals, ranking possible fire management strategies from the viewpoint of protecting endangered coastal Southern Californian wildlife. The model involves the direct and indirect effects of fire on four animal species of conservation concern (coastal cactus wren, California gnatcatcher, Stephens’ kangaroo rat, and Pacific pocket mouse) and five vegetation types (grass, coastal sage scrub, obligate seeding and resprouting chaparral, resprouting-only chaparral, and woodlands). Using historical fire records for the region, we predicted spatially-explicit fire frequencies and ignition probabilities. For these predictions, we simulated the location and extent of fires. Combining fire history and vegetation transition data from 1933 to 2003, we specified vegetation change probabilities under simulated fire regimes. Fire occurrence in a location altered habitat suitability, directly for each of the animal species and indirectly by changing the vegetative community. For some open-habitat species, such as the Stephens’ kangaroo rat and Pacific pocket mouse, fairly frequent fire is required to reduce the density of invasive grasses and herbs. For other species, such as the coastal cactus wren and California gnatcatcher, frequent fire destroys the mature coastal sage scrub on which these species depend. The model includes a management component, allowing us to rank fire management actions. Over a 50-year time horizon, we find that populations of California gnatcatchers and Pacific pocket mouse are highly variable, and the pocket mouse is particularly prone to decline, despite prescribed burns designed to boost population viability. California gnatcatchers were also likely to be extirpated in the model, with relatively small extirpation risks for the cactus wren and Stephens’ kangaroo rat. Despite conflicting requirements with respect to fire and differing life history traits among the four animals, we identified a beneficial strategy for our four target species, namely, controlling fire in coastal sage scrub.

Historic and Holocene environmental change in the San Antonio Creek Basin, mid-coastal California

Using a combination of pollen, non-pollen palynomorphs (NPPs) and charcoal particle stratigraphies from sediment cores from two sites, along with historical records, we reconstructed paleoenvironmental change in mid-coastal California. The San Antonio Creek section contains a discontinuous, Holocene-length record, while Mod Pond includes a continuous late Holocene record. Together the records allow for interpretation of most of the present interglacial. The longer record documents coastal sage scrub and chaparral dominated by woodland elements early in the Holocene to about 9000yr ago, a potential decline in woodland communities with drying conditions during the middle Holocene to about 4800yr ago, and an expansion of coastal sage scrub with grassland during the late Holocene. Evidence for climatic fluctuations during the last 1000yr at Mod Pond is equivocal, suggesting that the Medieval Climate Anomaly–Little Ice Age had modest impact on the Mod Pond environment. However, evidence of significant environmental change associated with cultural transitions in the 18th–19th centuries is stark. Introduction of non-native plants, establishment of cattle and sheep grazing, missionization of the native population, changes in burning practices during the Spanish period and enhanced cropping activities during North American settlement worked together to substantially modify the mid-California coastal landscape in about a century's time.

Effects of fragmentation on a distinctive coastal sage scrub bee fauna revealed through incidental captures by pitfall traps

Recent reports of pollinator declines have revealed a need to better document how anthropogenic disturbances and biogeography jointly influence wild pollinator communities. Here, we examine the effects of urbanization-induced habitat fragmentation on the native bee fauna inhabiting coastal sage scrub habitats of San Diego County, California, USA, a hotspot of bee biodiversity. Pitfall trapping in natural reserves and scrub habitat fragments yielded 70 native bee species or morphospecies, and revealed that bee species richness was lower in fragments than in reserves. However, fragments and reserves harbored bee assemblages similar in relative abundance, evenness, and community composition. Our samples yielded multiple species that are poorly represented in four of the leading institutions with collections of native bees from the southwestern United States, as well as 16 species represented only by specimen records from outside of San Diego County. Our results highlight the importance of continued efforts to document bee assemblages in under-studied regions with respect to their faunal distribution and basic taxonomy, as well as how they are impacted by anthropogenic disturbances such as habitat fragmentation. We also discuss the value of analyzing vouchered pitfall samples for non-target taxa captured incidentally.

Positive effects of non-native grasses on the growth of a native annual in a southern california ecosystem.

Fire disturbance is considered a major factor in the promotion of non-native plant species. Non-native grasses are adapted to fire and can alter environmental conditions and reduce resource availability in native coastal sage scrub and chaparral communities of southern California. In these communities persistence of non-native grasses following fire can inhibit establishment and growth of woody species. This may allow certain native herbaceous species to colonize and persist beneath gaps in the canopy. A field manipulative experiment with control, litter, and bare ground treatments was used to examine the impact of non-native grasses on growth and establishment of a native herbaceous species, Cryptantha muricata. C. muricata seedling survival, growth, and reproduction were greatest in the control treatment where non-native grasses were present. C. muricata plants growing in the presence of non-native grasses produced more than twice the number of flowers and more than twice the reproductive biomass of plants growing in the treatments where non-native grasses were removed. Total biomass and number of fruits were also greater in the plants growing in the presence of non-native grasses. Total biomass and reproductive biomass was also greater in late germinants than early germinants growing in the presence of non-native grasses. This study suggests a potential positive effect of non-native grasses on the performance of a particular native annual in a southern California ecosystem.

Influence of landscape-scale variables on vegetation conversion to exotic annual grassland in southern California, USA

In California, USA, coastal sage scrub (CSS) vegetation is being converted to exotic annual grassland, and several causes have been suggested. In order to investigate the importance of environmental variables in the conversion and recovery of CSS, particularly nitrogen deposition within the context of historical fire intervals, we employed an information theoretic approach. Prior studies have not assessed both conversion and recovery, and did not analyze nitrogen critical load for vegetation type conversion. We included measures of climate, topography, vegetation, land use, nitrogen deposition, and fire in our analysis, and found that 34% of CSS study sites were converted to exotic grassland between 1930 and 2009. Converted sites had higher nitrogen deposition with a critical load of 11 kg N ha−1 yr −1, also had shallower slopes, and were more west-facing. A smaller number of sites (24%) recovered to CSS, and these sites had about 2.5 times more CSS and 4.5 times less grassland in the surrounding landscape. CSS conservation and restoration efforts are most likely to be successful when focused on sites with <11.0 kg N ha−1 yr −1 and low invasion of exotic grasses. Analyses such as this that identify important threats may be useful in region-wide plans to conserve unique vegetation types.

Altered water and nitrogen input shifts succession in a southern California coastal sage community.

Vegetation-type conversions between grasslands and shrublands have occurred worldwide in semiarid regions over the last 150 years. Areas once covered by drought-deciduous shrubs in Southern California (coastal sage scrub) are converting to grasslands dominated by nonnative species. Increasing fire frequency, drought, and nitrogen deposition have all been hypothesized as causes of this conversion, though there is little direct evidence. We constructed rain-out shelters in a coastal sage scrub community following a wildfire, manipulated water and nitrogen input in a split-plot design, and collected annual data on community composition for four years. While shrub cover increased through time in all plots during the postfire succession, both drought and nitrogen significantly slowed recovery. Four years after the fire, average native shrub cover ranged from over 80% in water addition, ambient-nitrogen plots to 20% in water reduction, nitrogen addition plots. Nonnative grass cover was high following the fire and remained high in the water reduction plots through the third spring after the fire, before decreasing in the fourth year of the study. Adding nitrogen decreased the cover of native plants and increased the cover of nonnative grasses, but also increased the growth of one crown-sprouting shrub species. Our results suggest that extreme drought during postfire succession may slow or alter succession, possibly facilitating vegetation-type conversion of coastal sage scrub to grassland. Nitrogen addition slowed succession and, when combined with drought, significantly decreased native cover and increased grass cover. Fire, drought, and atmospheric N deposition are widespread aspects of environmental change that occur simultaneously in this system. Our results imply these drivers of change may reinforce each other, leading to a continued decline of native shrubs and conversion to annual grassland.

Soil nitrogen cycling is resilient to invasive annuals following restoration of coastal sage scrub

Southern California coastal sage scrub (CSS) is highly invaded by Mediterranean annual grasses and undergoing extensive restoration efforts. Exotic plant invasion alters ecosystem structure and function through plant-soil feedbacks that can be detrimental to native plants. Assessments of CSS restoration have focused on aboveground plant communities, while belowground effects have received less attention. We examined CSS soil resilience following restoration of native CSS species using ecosystem property divergence in restorations from an invaded state as a measure of exotic plant impacts. We hypothesized that exotic annual plants compete with native species for nutrients and change nutrient cycling, and exotic plant removal and native plant restoration would allow soil recovery under native plant-soil inputs. Nitrogen (N) cycling was resilient but not resistant to vegetation changes. Exotic annual plants increased N mineralization and nitrification but did not affect total soil carbon (C) and N and extractable phosphorus. Extractable N was reduced in invaded plots, and immediately increased following weeding. These changes suggest that exotic plants are directly competing with native plants for N. Impacts to N cycling were reversible after exotic plants were removed and native shrubs reestablished, which may have important implications for recovery of other ecosystems invaded by annual grasses.

Intra‐annual rainfall regime shifts competitive interactions between coastal sage scrub and invasive grasses

Changes in rainfall distribution, generally predicted by many climate models, can affect resource dynamics and ecosystem function. While little studied, intra-annual rainfall distribution may have particularly strong effects on competitive interactions. Here, we test whether increased rainfall event size and decreased frequency within a growing season can influence competitive dynamics related to the invasion of exotic annual grasses in California coastal sage scrub (CSS). We hypothesized that larger rainfall events and decreased frequency will increase the competitive ability of native CSS species: a deeper root system will permit greater water use during dry periods between pulses and enhance their resource depletion effect on more shallow-rooted grasses. We planted grass and CSS seedlings in an additive competition design under three rainfall treatments: frequent small events, infrequent large events, and infrequent small events. The first two treatments had the same total rainfall but different frequency, while the second and third treatments had the same frequency but different total rainfall. Rainfall treatment altered the competitive interactions between CSS and grasses. In the first year, the competitive effect of annual grasses on shrub seedlings was strongest under the frequent small rainfall regime where they reduced deep soil moisture and light. In year two, the established shrubs began to exert strong competitive effects on grasses, and these effects were strongest under the infrequent small rainfall regime (low total rain) where they reduced shallow soil moisture and decreased grass stomatal conductance. Results suggest that reductions in both rainfall frequency and total rainfall may be important to competitive interactions, and can alter plant community composition and invasion when species have different rooting depths and different responses to soil moisture.

Land use compounds habitat losses under projected climate change in a threatened California ecosystem.

Given the rapidly growing human population in mediterranean-climate systems, land use may pose a more immediate threat to biodiversity than climate change this century, yet few studies address the relative future impacts of both drivers. We assess spatial and temporal patterns of projected 21st century land use and climate change on California sage scrub (CSS), a plant association of considerable diversity and threatened status in the mediterranean-climate California Floristic Province. Using a species distribution modeling approach combined with spatially-explicit land use projections, we model habitat loss for 20 dominant shrub species under unlimited and no dispersal scenarios at two time intervals (early and late century) in two ecoregions in California (Central Coast and South Coast). Overall, projected climate change impacts were highly variable across CSS species and heavily dependent on dispersal assumptions. Projected anthropogenic land use drove greater relative habitat losses compared to projected climate change in many species. This pattern was only significant under assumptions of unlimited dispersal, however, where considerable climate-driven habitat gains offset some concurrent climate-driven habitat losses. Additionally, some of the habitat gained with projected climate change overlapped with projected land use. Most species showed potential northern habitat expansion and southern habitat contraction due to projected climate change, resulting in sharply contrasting patterns of impact between Central and South Coast Ecoregions. In the Central Coast, dispersal could play an important role moderating losses from both climate change and land use. In contrast, high geographic overlap in habitat losses driven by projected climate change and projected land use in the South Coast underscores the potential for compounding negative impacts of both drivers. Limiting habitat conversion may be a broadly beneficial strategy under climate change. We emphasize the importance of addressing both drivers in conservation and resource management planning.

Experimental Planting of Native Shrubs on Santa Cruz Island from Small Nursery Stock

The natural vegetation of Santa Cruz Island was severely disturbed by nonnative herbivores for well over a century. As the livestock and feral ungulates (primarily sheep, cattle, and pigs) were removed from the island over the last 30 years, many of the native plant communities began to recover naturally. Recovery has been extremely slow in other areas, especially where nonnative annual grasses and fennel (Foeniculum vulgare) dominate several hundred hectares that were under intense agricultural use (pastures and farmed lands). We experimentally tested the feasibility of speeding up the recovery process in a postagricultural area of the island's Central Valley using active restoration techniques. We assessed how weed control via herbicide application and planting of small nursery stock without irrigation might contribute to restoring natural plant assemblages in 3 different areas of the Central Valley (valley bottom, upper south-facing slope, and midsouth-facing slope). In February 2009, we planted the same 21 species of native plants in experimental plots with and without weed control at each location. In December 2009, we planted 28 species in adjacent plots after 2 seasons of weed control. We assessed natural recruitment in weeded and unweeded plots that were not planted. At all 3 locations, a single early season herbicide treatment prior to planting had strong positive effects on the survival, cover, and reproduction of planted natives compared to no herbicide treatment; the effects persisted and grew stronger in the second and third years. Repeated herbicide treatments over 2 years before planting did not result in any additional significant positive effects on native survival or growth compared to only one herbicide treatment. We saw virtually no natural recruitment of native shrubs in unplanted plots. We found that typical coastal sage scrub species performed best at the upper slope site and poorly at the valley bottom site. Grasses and shrubs tolerant of poorly drained soil did better in the valley bottom. We found that planting native species from small nursery stock without irrigation is effective for a wide range of grassland and coastal sage scrub species. All of the restoration techniques we used are cost effective and can be scaled-up to restore large areas of postagricultural lands.

Carnivore distributions across chaparral habitats exposed to wildfire and rural housing in southern California

Chaparral and coastal sage scrub habitats in southern California support biologically diverse plant and animal communities. However, native plant and animal species within these shrubland systems are increasingly exposed to human-caused wildfires and an expansion of the human–wildland interface. Few data exist to evaluate the effects of fire and anthropogenic pressures on plant and animal communities found in these environments. This is particularly true for carnivore communities. To address this knowledge gap, we collected detection–non-detection data with motion-sensor cameras and track plots to measure carnivore occupancy patterns following a large, human-caused wildfire (1134km2) in eastern San Diego County, California, USA, in 2003. Our focal species set included coyote (Canis latrans), gray fox (Urocyon cinereoargenteus), bobcat (Lynx rufus) and striped skunk (Mephitis mephitis). We evaluated the influence on species occupancies of the burned environment (burn edge, burn interior and unburned areas), proximity of rural homes, distance to riparian area and elevation. Gray fox occupancies were the highest overall, followed by striped skunk, coyote and bobcat. The three species considered as habitat and foraging generalists (gray fox, coyote, striped skunk) were common in all conditions. Occupancy patterns were consistent through time for all species except coyote, whose occupancies increased through time. In addition, environmental and anthropogenic variables had weak effects on all four species, and these responses were species-specific. Our results helped to describe a carnivore community exposed to frequent fire and rural human residences, and provide baseline data to inform fire management policy and wildlife management strategies in similar fire-prone ecosystems.

Occurrence of severe drought conditions in coastal southern California during the medieval climate anomaly inferred from pollen deposited in the Santa Barbara Basin since ∼AD 800

Pollen analyses of the upper 3.95 m from ODP Site 893B from the Santa Barbara Basin (SBB) provide proxy evidence for major climate-driven changes in the vegetation onshore between wAD 800 and AD 1800. Dominance of plant communities adapted to drought conditions of hot, dry southern California summers (chamise chaparral and coastal sage scrub) corresponds with increases in the duration and severity of western U.S. drought identified by Stine (1994) and Cook et al. (2004) and with MacDonald and Case's (2005) interpretation of more negative Pacific Decadal Oscillation variability during the Medieval Climate Anomaly. Extreme drought conditions occur during a period of multi-decadal drought recorded by the high resolution-Ti record between AD 1000-1100 (14C corrected) (Hendy et al., 2011). The shift toward wetter, cooler conditions atwAD 1400 (an increase inmoremesic oak and pine communities such as scrub oak and pine woodland chaparral, or more open grass woodland) coincides with temperature changes in the waters offshore. Correlative diatom and planktonic foraminifera deposited in the SBB during the Little Ice Age suggest increased seasonal sea surface temperature changes with cooler winters and warmer springs, respectively (Barron et al., 2010; Fisler and Hendy, 2008). The distinctive signature of the 19th and 20th century pollen assemblages reflects agricultural and residential impact on the natural vegetation of southern coastal California following European settlement.

Laboratory characterization of PM emissions from combustion of wildland biomass fuels

Particle emissions from open burning of southwestern (SW) and southeastern (SE) U.S. fuel types during 77 controlled laboratory burns are presented. The fuels include SW vegetation types: ceanothus, chamise/scrub oak, coastal sage scrub, California sagebrush, manzanita, maritime chaparral, masticated mesquite, oak savanna, and oak woodland, as well as SE vegetation types: 1 year, 2 year rough, pocosin, chipped understory, understory hardwood, and pine litter. The SW fuels burned at higher modified combustion efficiency (MCE) than the SE fuels resulting in lower particulate matter mass emission factor. Particle mass distributions for six fuels and particle number emission for all fuels are reported. Excellent mass closure (slope = 1.00,r2 = 0.94) between ions, metals, and carbon with total weight was obtained. Organic carbon emission factors inversely correlated (R2 = 0.72) with average MCE, while elemental carbon (EC) had little correlation with average MCE (R2 = 0.10). The EC/total carbon ratio sharply increased with MCE for MCEs exceeding 0.94. The average levoglucosan and total polycyclic aromatic hydrocarbon (PAH) emissions factors ranged from 25 to 1272 mg/kg fuel and 1.8 to 11.3 mg/kg fuel, respectively. No correlation between average MCE and emissions of PAHs/levoglucosan was found. Additionally, PAH diagnostic ratios were observed to be poor indicators of biomass burning. Large fuel type and regional dependency were observed in the emission rates of ammonium, nitrate, chloride, sodium, and potassium.

The invertebrate communities associated with a Chrysanthemum coronarium-invaded coastal sage scrub area in Southern California

The escape of ornamental plants is a main pathway of invasion into many ecosystems. Non-native plants can alter basal resources and abiotic factors leading to effects that ripple throughout an ecosystem. Invertebrates mediate these effects—responding quickly to abiotic and primary producer changes and, in turn, influencing other species. Invasions are of particular concern in the coastal sage scrub ecosystems of Southern California, where habitat loss and urban encroachment increase invasive species propagule sources and decrease native community resistance. The introduced annual Chrysanthemum coronarium (crown daisy) is a common invader with largely undocumented community-level effects. Our study tested the relationships between the invasive Chrysanthemum and a coastal scrub invertebrate community using a field study at the Tijuana River Estuary. We found similar or lower abundances and diversity of canopy fauna in the presence of Chrysanthemum. Community composition dramatically differed, however, in the presence Chrysanthemum, which was associated with higher abundances of dipterans, wasps and flower beetles, and lower abundances of hemipterans and thysanopterans than native shrubs. Differences in communities were consistent at the species- and order-levels, and were associated with the generally greater plant biomass and shadier conditions afforded by the natives. This study reveals that even a proportionally small amount of Chrysanthemum may shift the invertebrate community through alterations of abiotic properties and plant biomass. We recommend that Chrysanthemum be removed at the first sign of invasion or that spread is prevented since effects on the invertebrate community are dramatic and occur quickly.

Applying competition theory to invasion: resource impacts indicate invasion mechanisms in California shrublands

Despite widespread work documenting invasion, it remains a challenge to determine invasion mechanisms and incorporate them into invasive species management. Competition theory presents a strong model for evaluating the role of resource reduction and requirements in invasion. Additionally, alternative models suggest fluctuations in resources, niche differences, or non-resource priority effects are key factors determining invasion success. We propose a comparative framework that incorporates resource impacts of native and invasive species, performance in controlled invasion trials, and long-term natural invasion patterns to elucidate relative importance of these invasion mechanisms. To demonstrate this framework, we established monocultures of two representative native and two invasive plant species in Southern California’s coastal sage scrub (CSS), measured resource impacts (i.e., R*), and conducted invasion trials to test whether resource impacts predicted invasion success. We then related experimental results to field invasion patterns. Compared to exotic herbaceous species, native shrubs were associated with greater resource depletion of key resources: light, soil water (at multiple depths), and soil inorganic nitrogen (particularly at depth). In invasion trials, natives resisted invasion by the exotics, as resource depletion measures would predict. However, these results did not follow long-term natural invasion patterns indicating that these exotic species invade areas once dominated by native shrubland. Applying our results to the invasion framework, we conclude that disturbance, or a similar mechanism causing resources to fluctuate, is needed for exotics to invade CSS habitats. This resource-based comparative analysis of invasion mechanisms can point out important processes and help suggest effective management actions.

Comparing the Long-Term Impacts of Different Anthropogenic Disturbance Regimes on California Sage Scrub Recovery

California sage scrub (CSS) is a highly threatened vegetation community in coastal Southern California, 90 percent of which has been lost. Understanding CSS recovery is critical to its survival. This study compares long-term effects of grazing, cultivation, and mechanical disturbance in Southern California by tracking the extent of exotic grassland in two valleys in the Santa Monica Mountains over sixty years using aerial image analysis. Native shrubs returned to the grazed valley over one and one-half times faster than they did in the cultivated valley. Cultivation might result in a type conversion of CSS to exotic annual grassland that resembles a new steady state.

Anthropogenic impacts of irrigation on the arthropod community structure of a coastal sage scrub habitat in Los Angeles

We evaluated the effects of an irrigation system on the community structure of arthropods in a coastal sage scrub chaparral habitat. We placed 5 traps in the irrigated site and 5 traps in control site, for a total of 10 sticky traps (using Tree Tanglefoot) in an area adjacent to Ballona Wetlands in Los Angeles, California. The traps were set at the irrigated and non-irrigated site on Telegraph Weed for one week. Results showed that morphospecies richness, relative abundance, and taxonomic order representation were higher at the control site than at the irrigated site. Our study documents how anthropogenic impacts in a coastal sage scrub habitat can alter the community composition of invertebrates in the fringe habitats surrounding the last wetlands in Los Angeles County.

Prevalence and burden of two rickettsial phylotypes (G021 and G022) in Ixodes pacificus from California by real-time quantitative PCR

The western black-legged tick, Ixodes pacificus Cooley and Kohls, commonly bites humans in the far western U.S. In addition to transmitting Lyme borreliosis and anaplasmosis, it is a host of nonpathogenic bacteria as well as some of unknown pathogenicity. In this study, we report the detection, prevalence, and burden of 2 rickettsial phylotypes with unknown pathogenicity in I. pacificus ticks from 6 California counties using real-time quantitative PCR with phylotype-specific primers and probes. Prevalence of rickettsial phylotypes G021 and G022 from 247 I. pacificus ticks was 100% and 2.0%, respectively. The median burden of phylotype G021 was 7.3 per tick cell, whereas the burden of phylotype G022 was 0.8 per tick cell. The burden of phylotype G021 significantly differed between collection sites and between vegetation habitats. Ticks collected from the coastal sage scrub habitat of southern California had a lower burden of phylotype G021 when compared to central California oak woodland, northern California oak woodland, and mixed evergreen and ponderosa pine-oak habitats of northern California. No significant correlation was found between the burden of the phylotype G021 in the presence and absence of the phylotype G022 in I. pacificus, suggesting that the presence of these Rickettsia species do not interfere with each other in I. pacificus.

Exotic species display greater germination plasticity and higher germination rates than native species across multiple cues

Rapid germination or flexible germination cues may be key traits that facilitate the invasion of exotic plant species in new environments. We investigated whether robustness or plasticity in response to environmental cues were more commonly exhibited by exotic than native species during germination, evidenced by (1) exhibiting consistently greater germination rate under a variety of conditions (robustness), or (2) increasing germination rate more strongly than native species in response to favorable conditions (plasticity). We conducted growth chamber germination trials of 12 native and 12 exotic species common to coastal sage scrub, a shrub-dominated Mediterranean-type ecosystem in California. Time to germination and percentage germination were recorded in response to variation in three environmental cues: temperature, day length, and soil moisture. Exotic species, especially annuals, displayed consistently higher germination percentages and more rapid germination than native species. Exotic germination percentages also responded more strongly when conditions were favorable (warm temperatures and high soil moisture), and germinated earlier than natives when conditions were indicative of typical growing season conditions in Mediterranean ecosystems (short day length and cool temperatures). Exotic species had more rapid and prolific germination across a variety of environmental cues and in response to increased resource availability compared with native species, indicating both germination plasticity and robustness. These traits may enable colonization of novel environments, particularly if they allow exotic species to establish earlier in the growing season than native species, setting the stage for seasonal priority effects.